This invention relates to moisture curable organosiloxane coating compositions. More particularly, this invention relates to organosiloxane coating compositions that contain less than 5 weight percent of solvents or reactive diluents and are sufficiently low in viscosity for application by conventional coating techniques. The compositions cure rapidly to durable elastomeric coatings exhibiting excellent adhesion to a variety of substrates without generating hydrogen gas during storage or curing.
Conformal coatings containing polyorganosiloxanes are known in the art, and are useful as protective coatings for electronic components and circuit boards on which these components are mounted. Canadian Pat. No. 2,038,350 describes coating compositions that cure in the presence of ultraviolet radiation. The compositions contain a silicone resin with ethylenically unsaturated hydrocarbon radicals, an organic mercaptan, an acrylate monomer and a photoinitiator.
U.S. Pat. No. 4,568,566 which issued to Tolentino on Feb. 4, 1986 describes silicone resins useful as the curable ingredient in conformal coatings. The resins are curable by light in the presence of a photoinitiator or by heating in the presence of a heat-activated free radical generator. The resin contains a group of the formula C(R.sup.1).dbd.C(R.sup.1)--C(O)OR.sup.2, where R1 is H or a hydrocarbyl radical containing from 1 to 12 carbon atoms and R.sup.2 is a divalent unsubstituted or substituted hydrocarbon oxyalkylene radical.
Conformal coating compositions that cure by exposure to a combination of ultraviolet radiation and atmospheric moisture are described in U.S. Pat. No. 4,528,081, which issued to Lien and Nakos on Jul. 9, 1985 and U.S. Pat. Nos. 4,824,875 and 4,956,221, which issued to Gutek on Apr. 25, 1989 and Sep. 11, 1990, respectively.
Conformal coating compositions that cure by a platinum-catalyzed hydrosilation reaction are described in U.S. Pat. No. 5,082,894, which issued to VanWert and Houghtaling on Jan. 21, 1992. One advantage of the compositions described in these patents is their ability to cure in seconds or several minutes, thereby allowing the substrates to be handled shortly after being coated without damage to the coating.
The "dual cure" compositions described in the aforementioned patents to Lien et al. and Gutek cure rapidly in the areas accessible to ultraviolet radiation, however curing in the shadowed areas is considerably slower, and may require as long as 30 minutes or more, even at temperatures of 30.degree. C. and above, to develop a surface that i s not "tacky" due to insufficient curing. U. S. Pat. No. 5,008,301, which issued to Dennis and VanWert on Apr. 16, 1991 addresses this problem by including an organic peroxide as an ingredient of the curable composition to provide a relative rapid heat activated cure in the shadow areas. The residue and by-products resulting from decomposition of the peroxide can interfere with the operation of electronic components and cause corrosion of the substrate.
The preferred compositions described in the aforementioned VanWert and Houghtaling patent do not contain solvents or reactive diluents and cure in several minutes or less, depending upon the temperature. A shortcoming of coatings prepared using these compositions is their relatively poor strength and durability, which is believed due at least in part to the low molecular weight of the polydiorganosiloxane required to achieve a viscosity for the final curable compositions suitable for application to substrates by dipping or spraying.
Organosiloxane compositions that cure in the presence of atmospheric moisture by reaction of silicon-bonded hydroxyl groups with hydrolyzable groups such as alkoxy and carboxy groups are known, and have been used to prepare curable coating compositions. Alkoxy groups are preferred as the hydrolyzable group because the alcohol produced as a by-product of the curing reaction will not dissolve or corrode the materials used in solid state electronic devices and the substrates, such as copper plated printed circuit boards, on which these devices are mounted.
A curable organosiloxane composition preferred for use as conformal coatings for printed circuit boards and electronic devices contains polyorganosiloxanes with one silanol or at least two alkoxy groups at the terminal positions and a curing catalyst such as an organotin or organotitanium compound. When the polyorganosiloxane contains silanol groups the curable composition includes a silane or other relatively low molecular weight organosilicon compound containing at least two alkoxy groups per molecule.
The present inventors discovered that coatings prepared using solventless compositions containing conventional silanol-terminated polyorganosiloxanes, alkoxysilanes as curing agents and an organotitanium compound as the curing catalyst could be prepared. If the viscosity of the polyorganosiloxane was below about 1 Pa.s the composition could be sprayed, and the initially prepared compositions developed a tack-free surface in 5 minutes or less, which allowed the coated substrate to be handled. The shortcoming of these compositions is that the time required for the composition to form a tack-free surface increased substantially during storage of the composition.
U.S. Pat. No. 4,772,675, which issued to J. Klosowski and M. Meddaugh on Sep. 20, 1988 discusses the loss of curing ability over time that characterizes some moisture curable organosiloxane compositions containing alkoxy groups and titanium catalysts. The solution proposed in this patent is to use alkoxy functional polyorganosiloxanes wherein the alkoxy groups are present as di- or trialkoxysilalkylene terminal units. The terminal silicon atoms of these polyorganosiloxanes contain two or three alkoxy groups and are linked to the adjacent silicon atoms by an alkylene radical such as ethylene rather than by the oxygen atom characteristic of polyorganosiloxanes.
A preferred method for preparing the polyorganosiloxanes described in the Klosowski and Meddaugh patent involves reacting a polyorganosiloxane containing vinyl or other terminally unsaturated alkenyl radical with an "end-capping" organosilicon compound of the formula HR.sub.2 SiOSi(R.sub.2)CH.sub.2 CH.sub.2 Si(OR').sub.3. This organosilicon compound is, in turn, prepared by reaction of a disiloxane (HR.sub.2 Si).sub.2 O with a vinyltrialkoxysilane.
The present inventors discovered that when one or more of the curable polyorganosiloxanes prepared as described in the aforementioned U.S. Pat. No. 4,772,675 and the preceding paragraph, is used as the curable ingredient of a coating composition without substantial amounts of inorganic reinforcing fillers such as silica, hydrogen gas was evolved during curing and/or storage of the composition.
Because the curable composition is packaged in a closed container to avoid contact with atmospheric moisture and premature curing, the increase in pressure resulting from generation of a gas in this confined area can result in swelling or bursting of the container in addition to the potential for explosion associated with hydrogen gas.
Depending upon the viscosity of the curable composition and the rate of cure, the generation of hydrogen gas during curing causes foaming or the presence of voids in the final cured material. Either of these is undesirable in a coating.
It is therefore an objective of the present invention to provide coating compositions that do not contain solvents or reactive diluents, are sufficiently low in viscosity to be applied rapidly using conventional coating techniques such as brushing and spraying, can be stored for extended periods of time without any substantial decrease in cure rate or hydrogen generation, and develop a tack- and void-free surface in a sufficiently short time to permit subsequent processing of the coated substrate with only minimal delays.